What type of virtual switch must be configured to support communication between virtual machines and legacy systems on a physical server in a Windows 2016 Hyper-V environment?

The correct choice is an external switch because it facilitates communication between virtual machines and legacy systems running on a physical server. In a Windows 2016 Hyper-V environment, an external virtual switch connects virtual machines to the external network, allowing them to communicate with physical servers, legacy systems, and other devices outside the virtualized environment.

An external switch is configured to bridge the virtual network of the Hyper-V environment with the physical network, enabling seamless data exchange and connectivity. This is crucial when integrating virtual machines with older systems that may reside on the physical network.

The other switch types serve different purposes: a private switch allows communication solely among virtual machines on the same host and does not connect to the external network; a distributed switch manages the virtual networking across multiple Hyper-V hosts but does not directly facilitate communication with legacy systems; an internal switch allows communication between virtual machines and the Hyper-V host itself, but again does not enable connectivity to external legacy systems. Therefore, for the requirement of effective communication with legacy systems, the external switch is the most suitable option.